water Article Interdecadal Variability in Myanmar Rainfall in the Monsoon Season (May–October) Using Eigen Methods Zin Mie Mie Sein 1 , Irfan Ullah 2, * , Farhan Saleem 3,4 , Xiefei Zhi 2,5, *, Sidra Syed 6 and Kamran Azam 7   Citation: Mie Sein, Z.M.; Ullah, I.; Saleem, F.; Zhi, X.; Syed, S.; Azam, K. Interdecadal Variability in Myanmar Rainfall in the Monsoon Season (May–October) Using Eigen Methods. Water 2021, 13, 729. https://doi.org/ 10.3390/w13050729 Academic Editor: Scott Curtis Received: 26 January 2021 Accepted: 3 March 2021 Published: 7 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 College of International Students, Wuxi University, Wuxi 214105, China; dr.zin28@gmail.com 2 School of Atmospheric Science, Nanjing University of Information Science and Technology, Nanjing 210044, China 3 College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China; farhan@mail.iap.ac.cn 4 International Centre for Climate and Environment Sciences, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100049, China 5 Weather Online Institute of Meteorological Applications, Wuxi 214000, China 6 Institute of Peace and Conflicts Studies, University of Peshawar, Peshawar 25000, Pakistan; sidsyed.92@yahoo.com 7 Department of Management Sciences, University of Haripur, Khyber Pakhtunkhwa 22780, Pakistan; kamran.azam@uoh.edu.pk * Correspondence: irfan.marwat@nuist.edu.cn (I.U.); zhi@nuist.edu.cn (X.Z.) Abstract: In this study, we investigated the interdecadal variability in monsoon rainfall in the Myanmar region. The gauge-based gridded rainfall dataset of the Global Precipitation Climatology Centre (GPCC) and Climatic Research Unit version TS4.0 (CRU TS4.0) were used (1950–2019) to investigate the interdecadal variability in summer monsoon rainfall using empirical orthogonal function (EOF), singular value decomposition (SVD), and correlation approaches. The results reveal relatively negative rainfall anomalies during the 1980s, 1990s, and 2000s, whereas strong positive rainfall anomalies were identified for the 1970s and 2010s. The dominant spatial variability mode showed a dipole pattern with a total variance of 47%. The power spectra of the principal component (PC) from EOF revealed a significant peak during decadal timescales (20–30 years). The Myanmar summer monsoon rainfall positively correlated with Atlantic multidecadal oscillation (AMO) and negatively correlated with Pacific decadal oscillation (PDO). The results reveal that extreme monsoon rainfall (flood) events occurred during the negative phase of the PDO and below-average rainfall (drought) occurred during the positive phase of the PDO. The cold phase (warm phase) of AMO was generally associated with negative (positive) decadal monsoon rainfall. The first SVD mode indicated the Myanmar rainfall pattern associated with the cold and warm phase of the PDO and AMO, suggesting that enhanced rainfall for about 53% of the square covariance fraction was related to heavy rain over the study region except for the central and eastern parts. The second SVD mode demonstrated warm sea surface temperature (SST) in the eastern equatorial Pacific (El Niño pattern) and cold SST in the North Atlantic Ocean, implying a rainfall deficit of about 33% of the square covariance fraction, which could be associated with dry El Niño conditions (drought). The third SVD revealed that cold SSTs in the central and eastern equatorial Pacific (La Niña pattern) caused enhance rainfall with a 6.7% square covariance fraction related to flood conditions. Thus, the extra- subtropical phenomena may affect the average summer monsoon trends over Myanmar by enhancing the cross-equatorial moisture trajectories into the North Atlantic Ocean. Keywords: summer monsoon rainfall; interdecadal variability; AMO; PDO; Myanmar 1. Introduction Rainfall variability is among the basic indicators of climate and water cycle changes in a region [1]. Anomalous changes in rainfall dynamics may cause hydro-meteorological hazards such as flood, drought, and storms [2,3], thus ultimately resulting in loss of human Water 2021, 13, 729. https://doi.org/10.3390/w13050729 https://www.mdpi.com/journal/water